This study will investigate progressive hearing loss and the correlated cellular changes in the auditory pathway with advancing age. The work will be complementary to an ongoing, funded NIH Program Project based in the Principal Investigator's department. This Program Project is devoted to the analysis of a variety of neural changes with age in a well-defined population of aging rhesus monkeys (Macaca mulatta). The presently proposed research program will utilize the Program Project animals to carry out correlated functional (evoked potential) and structural (fight and electron microscopic) studies of the aging monkey auditory system. Of all available animal models of human auditory decline with age (presbyacusis), the rhesus monkey provides the most apt model, by virtue of its lifespan, functional and structural similarity to the human auditory system, and what is known of its auditory aging. The present opportunity to study auditory aging in this monkey population is an unparalleled one, and is not likely to be available again in the foreseeable future. Tone-evoked brainstem auditory potentials will be studied in each animal, immediately prior to sacrifice by the Program Project, in order to assess hearing status. Following sacrifice, detailed morphological studies will be carried out on the cell population's within the superior olivary complex, inferior colliculus, and auditory cortex; in addition, the losses of receptor cells of the cochlea and spiral ganglion cells will be evaluated. Using light and electron microscopic immunocytochemistry, as well as conventional cytomorphological techniques, studies will aim to shed light on human presbyacusis by analyzing peripheral and central cell loss, the genesis and subsequent tempo and severity of cellular degeneration, and declines with age in immunogenicity (using, e.g., calcium binding protein, cytoskeletal, and GABA antibodies). Almost all of the analyses will represent the first such studies of presbyacutic changes in this important animal model.